View Full Version : 14.5 midlength/failure to lock back
I have a BCM 14.5 mid length upper on a LMT lower. When I first put the rifle together, I tested out H, H2, and H3 buffers with XM193. All three locked the bolt back consistently. I then tested them with Remington UMC .223. All three locked back, but the H3 felt "sluggish." Based on similar findings I had read here, I settled with the H2. I then ran an additional 60 rounds of the UMC single shot through the rifle to confirm that the H2 would function with the weaker ammo. Zero failures.
Fast forward to yesterday. I take the rifle out and all of a sudden the bolt inconsistently locks back with XM193 ammo. The magazine ran empty a total of around 40 times. Of those 40 times, there were two confirmed and one possible failures of the bolt to lock back (buddy was firing the weapon and could not remember if it locked back or not).
I have checked the gas system and there do not appear to be any leaks. I only had failures with one mag, but it was a brand new PMag. That same mag was used in my buddy's rifles with zero failures. The only thing I can think is that the rifle was a little dry on yesterday's range trip. Would a little less lube make so much of a difference that the gun can't even run hot ammo all of a sudden?
I would prefer the rifle to be able to cylce in a pinch even if it is bone dry. Should I continue to try to trouble shoot this issue with the H2 or should I just drop down to an H buffer?
Edit to add: The bolt was still lubed, it just wasn't dripping.
Robb Jensen
03-06-11, 11:50
UMC should only be trusted to run a gun if it's suppressed. Yes how much lube you have on the carrier, cam pin, bolt do make a difference.
Thanks for the quick reply. I was only using the UMC to test the buffer with weak ammo. I pretty much only shoot XM193 and Speer Gold Dot. What I can't figure out is why it consistently locked back with the weaker ammo (UMC) and then all of a sudden will not lock back with XM193.
I run mine wet. But you could easily test the conditions and see if that was the issue. Lube it up and run XX number rounds of the said ammo through it with the buffer in question. Dry it off and repeat the test to see if the condition appears. Do the same with the H buffer and see what you get.
How dirty is the rifle? Dirty & dry is going to bring reliability down.
....
Robb Jensen
03-06-11, 12:15
Thanks for the quick reply. I was only using the UMC to test the buffer with weak ammo. I pretty much only shoot XM193 and Speer Gold Dot. What I can't figure out is why it consistently locked back with the weaker ammo (UMC) and then all of a sudden will not lock back with XM193.
What make of BCG?
Did you add a BAD lever or similar device?
How many different mags tested?
What make of BCG?
Did you add a BAD lever or similar device?
How many different mags tested?
1. BCM
2. No.
3. I only had two with me yesterday. Had failures with one (but also put more rounds through it). However, I have used this mag in the past without failures and it runs fine in other rifles. The mag is an OLDER version PMag with less than 500 rounds through it. No clue how many mags have been used in the rifle altogether. Probably 6-10.
Part of the pain in the ass about this is that there is only a failure about 1/20 times the mag runs empty. I'm gonna have to use a case of ammo just to be able to function check. :suicide:
Correction: I checked the mag. It is an older revision Pmag (pre-09 markings). Why the hell is Brownells still selling old Pmags? I thought all those disappeared during the Obamarama. The other mag I received at the same time (and had no failures with yesterday) is marked "10."
Now that I know this, I am leaning toward a magazine issue. I have had issues in the past with older PMags and LMT lowers.
Any indication of gas leakage around the base of the carrier key? FTLB was the first indication of my rifle's issue with a loose key a few months back.
Any indication of gas leakage around the base of the carrier key? FTLB was the first indication of my rifle's issue with a loose key a few months back.
Negative.
Robb Jensen
03-06-11, 12:48
1. BCM
2. No.
3. I only had two with me yesterday. Had failures with one (but also put more rounds through it). However, I have used this mag in the past without failures and it runs fine in other rifles. The mag is an OLDER version PMag with less than 500 rounds through it.
Part of the pain in the ass about this is that there is only a failure about 1/20 times the mag runs empty. I'm gonna have to use a case of ammo just to be able to function check. :suicide:
Correction: I checked the mag. It is an older revision Pmag (pre-09 markings). Why the hell is Brownells still selling old Pmags? I thought all those disappeared during the Obamarama. The other mag I received at the same time (and had no failures with yesterday) is marked "10."
Now that I know this, I am leaning toward a magazine issue. I have had issues in the past with older PMags and LMT lowers.
Lube it well.
Try more newer mags. Check with one round to each ensuring it'll lock back when empty when firing.
If it still fails to lock back when empty try another known good bolt carrier (but your current bolt, FP, Cam pin).
Thanks for the advice guys. I'll try to make it back to the range today. I'll update whenever I have results.
I switched to an H Buffer yesterday. Like you... when my 14.5 middy was new it ran everything.... even on an H3 buffer. I ran an H2 for about 1000 rounds.
But now it's settled into not locking back 100% with .223 ammo.
The gun still cycles 100%... smooth, and postive ejection.
What was the weather temperature when this happened?
I switched to an H Buffer yesterday. Like you... when my 14.5 middy was new it ran everything.... even on an H3 buffer. I ran an H2 for about 1000 rounds.
But now it's settled into not locking back 100% with .223 ammo.
The gun still cycles 100%... smooth, and postive ejection.
That's a little disappointing. I could feel a difference with the H2 buffer. I was hoping the rifle would loosen up to be able to run a H3 (the H3 had noticeably less recoil when I was testing). My 10.5 started out unable to lock back on a H3 (still cycled reliably) and got to where it could run perfectly with one after about 1000 rounds. Maybe the shorter barrel is more likely to get early gas port erosion? :confused:
What was the weather temperature when this happened?
Southern Arizona (warm and sunny).
Okay, here's the update, guys. Just got back from the range.
I shot XM193 with the H2 buffer and newer 2010 PMags. I did not clean the rifle or add any lube from yesterday. No malfunctions. The bolt locked back 52 out of 52 times. This was following both single shots and strings of fire.
I am going to chalk this up to some sort of tolerance stacking issue between the older magazine and the LMT Lower. I'll keep my eye on it, to see if it pops back up. For now I'm going to say that the case is closed.
What was the weather temperature when this happened?
That's the only variable I can think of.
It was hot as hell the first time I started messing with my upper. I'm talking well over 100 degrees when we shot all the video and stuff.
That's the only variable I can think of.
It was hot as hell the first time I started messing with my upper. I'm talking well over 100 degrees when we shot all the video and stuff.
I bought my upper last fall, but didn't get around to putting the gun together until winter. Pretty sure I was wearing a jacket when I tried all the different buffers. :confused:
I'm glad you seem to have the problem isolated to the Pmags being the possible culprit. I have a similar situation with a different set up but reading this introduces my magazines into the fold as the possible solution.
Best of luck and keep us posted.
Robb Jensen
03-06-11, 20:46
Things that control bolt lock back when empty:
Bolt catch...it has to have the full range of travel.
Bolt catch weight...adding a bolt catch device which adds weight to the bolt catch can make an otherwise good functioning bolt catch stop functioning. Adding weight to it with an aftermarket device slows its upward travel.
Magazine follower travel and mag spring pressure...... Follower has to rise high enough to lift the bolt catch. Mag spring has to lift the follower enough to catch to bolt catch in the time the BCG is to the rear. Follower might be in 100% condition but if the mag spring can't get it to were it needs to be at the right time it's useless.
Ammo...if the ammo is so damn weak that it's not making the bolt carrier move ALL the way to the rear where it contacts the very interior end of the receiver extension than the bolt may not lock back nor allow the next round in the magazine to feed into the chamber.
The follower on the older P mags came up higher in the magazine than the newer ones. If the follower is Foliage green you have one of the newer mags and it can be a problem getting the last shot to lock open.
I'm not sure why Magpul decided to fix something that wasn't broken.
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